Electric circuit breakers for low voltage and high current applications have not heretofore been fabricated on high speed efficient assembly lines. The large number of breaker ratings for each frame size generally require a correspondingly large number of different breaker designs for each rating. Because of the variety of different parts required for each of the breaker ratings, it is difficult for a single assembly line to efficiently assemble more than a few breaker designs without substantial changeover in parts, tools and test procedures. A further drawback to efficient high speed breaker assembly is the stringent requirement that each breaker be individually tested for calibration. This is accomplished by applying a test current above the steady state rated current and determining whether the breaker trips within a predetermined time interval. If the breaker successfully trips within the time interval, the breaker is then forwarded along the assembly line to the next step in the assembly process. If the breaker fails to trip within the proper time, an adjustment is made to correct breaker tripping before the breaker can proceed along the assembly process. The number of breakers successfully passing the trip test, i.e. tripping within the required time interval, in relation to the total number of breakers tested, is defined as the "yield". For a 150 amp industrial type E-Frame breaker assembly line for example, a typical yield value should be in excess of seventy percent.
Another factor that effects the speed and efficiency of the existing breaker assembly process is the engagement of the operating springs within the operating mechanism assembly. The spring is engaged in an uncharged or un-stressed condition on the operating spring support pin and is then connected with the operating handle yoke by the use of a special tool. The operator first engages the hook of the operating spring by inserting the tool through an opening in the top of the handle yoke and further elongating the spring to move the hook back through the opening to engage a web on the handle yoke crosspiece. Since there are two operating springs involved, some valuable assembly time is involved even by skilled operators.
A further obstacle to an efficient high speed breaker assembly process is the difficulty encountered in assembling the contact spring sub-assembly to the contact arm carrier against the bias of the contact spring.
A time consuming polishing process is required on the latch system's secondary latch surfaces. The polishing is required to minimize the amount of tripping force that must be applied to overcome the bias of the operating spring and the static friction of the contacting latch surfaces. Although the polishing can be done in a separate pre-assembly process without affecting the actual circuit breaker assembly operation, it has been determined that the variation in the "trip force", that is the amount of force that must be applied to the trip bar to overcome the latch spring bias and latch surface friction, depends to a certain extent upon the polishing operation. A typical value of the coefficient of fricition for an unpolished secondary latch surface is 0.5 where the value for a highly polished secondary latch surface can be as low as 0.1. The primary and secondary latch surfaces are fabricated from stamped metal parts which exhibit a burr on the edge of one surface and a die roll on the edge of the opposite surface. With secondary latch mating surfaces, the burr edge surface can result in variable frictional forces even after polishing.
One example of an industrial type E-frame circuit breaker employing primary and secondary latches is given within U.S. Pat. No. 3,605,052 in the names of Herbert M. Dimond et al. This breaker employs a pivotally mounted rectangular latch plate having a rectangular aperture cut through the plate to support the end of the cradle under a forward edge of the latch plate aperture when the breaker is in a "latched" condition. This forward edge comprises the primary latch surface for this breaker design and is "shaved" to insure a flat surface. Both the cradle and the latch are fabricated from a stamping operation followed by a shaving operation to flatten and smooth the surface of the cradle and the latch aperture to maintain a low trip force between the cradle and primary latch surfaces. For a good description of the shaving operation see pages 116-118 in the publication entitled "Advanced Diemaking", McGraw Hill Book Company 1967 Edition, New York, N.Y. The secondary latch surface for the aforementioned E-frame breaker comprises the rear surface of the latch plate which retains a rolled pin connected to the trip bar. Since the primary latching forces provided by the operating spring are much greater than the secondary latching forces provided by the lighter secondary latch spring, the effect of friction is substantially critical with respect to release between the secondary latch surface and the trip bar rolled pin. The rolled pin is formed from a high carbon steel and is rounded over to provide a smooth contact surface with the secondary latch surface and the latch plate is oriented to provide the smooth stamped surface with the smooth die rolled edges facing the trip bar rolled pin.
An early attempt to reduce friction between latching surfaces is described within U.S. Pat. No. 4,119,935 entitled "Circuit Interrupter Including Low Friction Latch". This patent describes latching surfaces having a rough and smooth portion resulting from the metal stamping operation and disposes the latching surface so that only the smooth portions of the latching surfaces are in contact.
The purpose of this invention is to provide a circuit breaker design and a method of assembly which substantially overcomes the aforementioned obstacles to result in an efficient high speed circuit breaker assembly process.